1. Field of the Invention
This invention relates to a multi-plate frictional engagement apparatus that obtains a torque by applying a high pressure to an opposed surface of a wet clutch, a brake friction material, lock-up clutch or the like, while immersing it in an oil, in an automatic transmission (may be referred to as “AT” hereafter) of an automobile or the like. Particularly, this invention relates to a multi-plate frictional engagement apparatus capable of improving heat resistance without increase of a pack amount (axial length).
2. Description of the Related Art
In recent years, it is requested to improve heat resistance more in a multi-plate frictional engagement apparatus such as a wet clutch, a wet brake or the like used in an automatic transmission or the like. If enlarging a thickness of a metal plate, which has a frictional surface adhered with a wet friction material, a temperature on the frictional surface is lowered by heat absorption by the metal plate and the heat resistance is improved. However, the multi-plate frictional engagement apparatus occupies more space (pack amount) by that much and it goes against a request for downsizing or reducing the space of the automatic transmission or the like.
In view of that fact, a first patent publication (Japanese Laid Open Patent Publication No. H8-326776) discloses a technique for a multi-plate frictional engagement apparatus. In the apparatus, a wet frictional material as a heat insulator is not attached to an external tooth plate at a nearest side to a piston. On the other hand, the wet frictional materials are attached to opposite surfaces of an internal tooth plate at a nearest side to the piston. Thereby, a heat is released from a back surface of the external tooth plate at the nearest side to the piston, thereby increasing a heat capacity so as to improve a heat resistance of the multi-plate frictional engagement apparatus.
A second patent publication (Japanese laid Open Patent Publication No. H10-78051) discloses another technique for a frictional engagement apparatus. The apparatus is a single face frictional engagement apparatus in which wet friction materials are adhere to just one surface of each of all external tooth plates and all internal tooth plates. In the apparatus, there may be a problem that a temperature of part of the external tooth plate or the internal tooth plate rapidly rises due to an eccentric frictional engagement such as an inner side contact or an outer side contact. In such case, the external tooth plate and the internal tooth plate are thermally distorted. Therefore, in order to prevent such problem, notches are provided along an overall inner circumference of the external tooth plate and an overall outer circumference of the internal tooth plate so as to reduce the thermal distortion.
A third patent publication (Japanese Laid Open Patent Publication No. H10-169681) discloses still another technique for a frictional engagement apparatus. The apparatus is also a single face frictional engagement apparatus. The apparatus has a friction material fixed on an external tooth plate, while forming grooves on the function material so as to be inclined along a rotational direction of the external tooth plate. Moreover, the apparatus has a friction material fixed on an internal tooth plate, while forming grooves on the friction material so as not to be inclined. Consequently, oil is hard to be discharged from a frictional surface of the external tooth plate, thereby preventing eccentric location of a disc or increase in a drag torque.
However, in the invention described in the first patent publication, the heat is released only from the back surface of the external tooth plate at the nearest side to the piston. Therefore, heat discharge is not sufficient and there is a limit in improving the heat resistance. In the invention described in the second patent publication, it is necessary to provide the notches along the overall inner circumference of the external tooth plate and the overall outer circumference of the internal tooth plate. Therefore, there may be a problem that a machining process is complicated and causes production costs to increase. Moreover, since a lining is cut by the notches, there may be another problem or adverse effect that a wear resistance of the lining is deteriorated. In the invention described in the third patent publication, the grooves need to be formed on the external tooth plate and the internal tooth plate in a different manner. Therefore, there may be similar problems as described in the second patent publication.